Typically one thinks of the key aspect of interferometer design as an attempt to match the path lengths of the sample and reference arms. In the time domain, one only observes an interference envelope when these two paths are closely matched. The length of the reference arm was typically modulated to correspond to various depths within the tissue. In state of the art frequency domain systems, the reference is usually placed at fixed distance relative to the tissue and the maximum difference between the two arms is limited by the sampling of k-space. With a finer line width and greater number of samples, one can observe a greater distance away from the zero delay.
The attempt to match path lengths is only one aspect of the actual problem one is trying to solve when performing white light interferometry. The problem can be more generically stated: In order to observe interference between the two paths, the phase difference between the two paths must vary across the spectrum in a manner which can be observed by a detector. When the light from the reference path interferes with the light from the sample path, the phase of interference may be detected if:                1. The optical bandwidth simultaneously superimposed upon a location of the detector is sufficiently narrow to produce an interference that is substantially invariant during the measurement time (coherence length).        2. The phase difference from optical frequency to optical frequency changes at a slow enough rate that it may be sampled by the detector.        
For spectrally resolved detection, the rate of phase difference across the spectrum must vary slowly enough that the phase difference is essentially constant across each detector element. For temporally resolved detection, the rate of phase difference across the spectrum must vary slowly enough that the phase difference is essentially constant during the measurement time period.
The intensity observed on a detector at the output of an interferometer is dependent on the phase difference achieved in the two paths. When light propagates a distance, z, the phase, (Φ), gained across the spectrum can be described as a linear ramp with respect to k, the wavenumber in the medium traversed (ΔΦ=k·z) where k˜1/λ. Restated: When light traverses a distance in the sample, the short wavelengths gain more phase than the long wavelengths do. If light in the two arms of the interferometer travels the same distance, it acquires the same phase ramp. If the interference of these two beams is observed on a spectrally resolved detector, a constant intensity is observed across the spectrum. A modulated interference is not observed because the phase difference between the two arms is constant. Now for example, consider light that has traveled a shorter distance in one arm, and has therefore acquired a less steep phase ramp. If the interference of these two beams is observed on a detector, a sinusoid of intensity is observed across the spectrum, because the phase difference varies linearly across the spectrum. Matching the distances in the two arms ensures that difference in the two phase ramps is not so large that the sinusoid is of too high a frequency to be observed.
Other ways exist to introduce a phase ramp across the spectrum. For example, the commonly used ‘Fast-Fourier-Domain Delay Line’ or ‘Rapid Scanning Optical Delay Line’ introduced a variable phase ramp on the reference path by spreading the spectrum with a diffraction grating, and then reflecting it off of a minor with a changing angle. The effect was to change the phase of short wavelengths to a relatively high extent, while changing the phase of long wavelengths relatively little. By this mechanism, the phase ramps of the two arms could be matched (for a particular depth in the tissue) and a deep time domain scan could be achieved with little translation of a minor surface. The structure of the Fast Fourier Domain Delay line was largely borrowed from prismatic or grating implementations of the pulse stretcher—compressor designs used for amplifying short pulse lasers. A similar Fourier Domain Delay line was developed for use with time domain OCT using a pair of stretched chirped fiber Bragg gratings.
A Bragg grating is a reflective optical device created by introducing many small reflections distributed through a volume which interfere constructively to create a precisely tuned reflectivity. The chirped Bragg grating introduces an arbitrary phase delay structure in a reflected beam by effectively making each point along the length of the structure reflective to a different wavelength. A fiber Bragg grating (FBG) is a reflective structure created within the volume of an optical fiber by introducing index of refraction variation along its length, often by exposing the glass to patterned ultraviolet radiation. Chirped fiber Bragg gratings, in which the grating period has a linear variation, are commonly used in short pulse amplification as well as dispersion compensation in optical communication networks. Bragg gratings may also be created in bulk optic materials, and in other waveguide structures, such as silicon planar waveguides, which are commonly used in integrated optical components. Bragg gratings may also be created by dynamic modulation, such as within an acousto-optic modulator.
The ‘common path’ or Fizeau interferometer configuration has the advantage in optical coherence tomography (OCT) that there is very little opportunity for polarization or dispersion mismatch between the arms of the interferometer. With a traditional Fizeau interferometer used in frequency domain OCT, the distance between the reference surface and the imaged surface determines the frequency of modulation across the spectrum that is detected. As the distance becomes larger, the modulation frequency increases. The spectrum modulation frequency is an important factor in the design of an OCT system, determining the number of detector elements required in spatially encoded frequency domain (spectral domain) OCT system, and the temporal detection frequency required in a time encoded (swept source) frequency domain OCT system. The difficulty in placing the reference surface at an ideal distance relative to the biological sample, due to mechanical constraints, is an important drawback that prevents the Fizeau configuration from being used in many instances. The invention described herein proposes a solution which allows a Fizeau interferometer topology, with a physical reference structure that can be located a relatively large distance away from an imaged surface, while keeping spectral modulation frequency at a measurably low rate.